This invention is generally directed to a method and apparatus for dispersal of aggregates in a fluid medium. The present invention employs a sonic or ultrasonic device to efficiently breakup particle agglomerates by driving the ultrasonic signal over a small range of frequencies around the acoustic slow wave frequency of the saturated agglomerate. At this frequency, the fluid vibrates out of phase with the solid and is forced out through the pore structure in the agglomerate, exerting stresses on the aggregate frame which cause breakup.
Cross reference is made to the following applications filed on Oct. 30, 2000: U.S. Ser. No. 09/699,703 entitled xe2x80x9cProcess And Apparatus For Obtaining Ink Dispersing By Subjecting The Liquid Inks To An Ultrasonic Or Sonic Signalxe2x80x9d, U.S. Ser. No. 09/699,862 entitled xe2x80x9cMethod For Improving Oil Recovery Using An Ultrasound Techniquexe2x80x9d, U.S. Ser. No. 09/699,871 entitled xe2x80x9cA Method For Removing Trapped Impurity Aggregates From A Filterxe2x80x9d, U.S. Ser. No. 09/699,882 entitled xe2x80x9cUltrasonic Cleaner And Toner Agglomerate Disperser For Liquid Ink Development (LID) Systems Using Second Soundxe2x80x9d, U.S. Ser. No. 09/699,804 entitled xe2x80x9cMethod For Dispersing Red And White Blood Cellsxe2x80x9d, and U.S. Ser. No. 09/699,876 entitled xe2x80x9cUltrasonic Drying of Saturated Porous Solids Via Second Soundxe2x80x9d.
Liquid electrostatic developers having chargeable toner particles dispersed in an insulating nonpolar liquid are well known in the art and are used to develop latent electrostatic images. Ideally, such liquid developers should be replenishable in the particular equipment in which they are used. In general, high solids concentration toners are used for replenishment because relatively low concentrations (e.g., in the range of 10 to 15% by weight solids) result in greater liquid build-up in the equipment, which then must be removed and disposed of as hazardous waste. Thus, it is desirable to initially use a toner containing less liquid, and to maintain the working source located within the equipment, thereby minimizing the undesirable accumulation of carrier liquid in the equipment. When toners are present in the liquid developer in more concentrated form, however, they become difficult to redisperse in the carrier. For example, aggregates may be formed. This can cause serious problems in the replenishment of the liquid developer in the equipment being use. It has been known to use high shear forces between two closely spaced cylindrical surfaces to dissociate liquid toner particles as disclosed in U.S. Pat. Nos. 5,004,165, 5,048,762, 5,078,504, and 5,492,788.
In printing applications these aggregation effects result in grainy images, poor coating uniformity, and poor image to image uniformity and image quality. Breaking up aggregates will result in better image quality. Dispersed particles in inks are subject to many effects that lead to coagulation, limiting shelf life. The liquid-based inks (LID, CEP, and any liquid-based dispersion of small particles) tend to coagulate if left on the shelf for long periods before use. Particles tend to settle under the influence of gravity, producing a sludge layer at the bottom of the container. Brownian motion of the particles due to thermal effects tends to bring particles into contact with one another, leading to coagulation and settling. Charge variations, especially in systems having both sign particles, leads to settling of ink particles. Therefore, it is desirable to have a method and apparatus to readily disperse the particles.
It is desirable to have a method and apparatus to obtain good color saturation. It is known that the color saturation, or chroma level, that can be achieved by color toners consisting of color pigments dispersed in a transparent binder is influenced to a large degree by the completeness of the dispersion of the pigments. Aggregated pigments tend to produce toners with washed-out or less bright colors than those achieved with well-dispersed pigments. On the other hand, it is difficult to achieve good dispersion with color pigments. This is due to the strong van der Waals forces that exist between these pigments, leading to strong, hard to disperse, aggregates.
The number of color pigments that can be used in the manufacture of EA toner is severely limited. In many cases one is forced to use pigments that have unwanted optical absorptions (i.e., absorb light at frequencies we don""t want), giving colors that aren""t exactly what we want, because we can disperse these pigments with the techniques at hand (e.g., sonicators, microfluidizers, Brinkman Polytrons are cited for example in U.S. Pat. No. 5,482,812 to Hopper et al. patent). There are many other pigments we would like to use, either for general application because their absorption spectrum is more in line with the color we want, or for custom purposes (e.g., to match the specific colors desired for a specific account: Kodak orange, John Deere green, etc.). Indeed, the color gamut of our copiers and printers is limited, not by the optical properties of the pigments available, but by the available pigments we can disperse. For example, there are 13 commonly available blue pigments, of which we typically use 1; there are 30 commonly available yellow pigments, of which we typically use 4; and there are 62 commonly available red pigments, or which we typically use 2. The other pigments are not used for several reasons. There may be health problems associated with their use; there may be problems associated with their effects on toner charging or tribo. However, these are not the primary characteristic that limits pigment use. Pigments are primarily rejected because their high adhesion characteristics make them too hard to disperse.
As noted above, pigment particles are found to be especially hard to disperse due to strong adhesion forces between the particles. This turns out to be a fundamental result of their bright color. The vivid color is a result of strong light absorption over a frequency band, i.e., a high imaginary part of the dielectric constant over a range of frequencies. The Lifshitz theory of van der Waals forces (discussed in Abrikosov, Gorkov, and Dzaloshinski, Methods of Quantum Field Theory in Statistical Physics) shows that the strength of the force between two bodies 1 and 2 is proportional to:
FvdWxe2x88x9d∫dxcfx891∫dxcfx892[Im(∈(xcfx891))Im(∈(xcfx892))/(xcfx891+xcfx892)]dxcfx891dxcfx892.xe2x80x83xe2x80x83(1)
where Im(∈(xcfx891)) is the imaginary part of the frequency-dependant dielectric constant of pigment particle i, and xcfx89=2xcfx80f, and f is the frequency of light. The term Im(∈(xcfx891)) is the term that gives absorption of light at certain frequencies, resulting in color. Thus, colorful materials which have high Im(∈(xcfx891)), such as pigments, also tend to be sticky materials because of their high van der Waals forces, as indicated via Eq. (1). As a result, all color pigments tend to be especially difficult to disperse by their very nature.
A somewhat older model of van der Waals forces is due to London (1930). While this model is not as accurate as the Lifshitz (1955) model (mentioned above), it can readily be used to predict pigment-pigment cohesion, and it""s predictions are generally in agreement with experimental trends. In this model the van der Waals force between two bodies is proportional to the atomic polarizability per unit volume of each of the constituent elements. Polarizability per unit volume is a dimensionless number, independent of the unit system utilized. A simple model that accounts for many of the van der Waals adhesion properties of pigments is obtained by assigning a unique atomic polarizability to each element, regardless of the type of its molecular bonding in a compound. These polarizabilities can be obtained from published tables, or via simple least squares fitting procedures using published tables of molecular polarizabilities (CRC Handbook of Chemistry and Physics, 80th Edition). Similarly, elemental atomic volumes can be obtained from published tables, or via fitting to published pigment densities (NPIRI Raw Materials Data Handbook, vol. 4, Pigments). From this analysis we can make predictions of the relative strength of van der Waals cohesion between pigment particles. The van der Waals cohesion force should scale as the square of the molecular polarizability per unit volume.
An example of the difficulty of dispersing color pigments occurs in the selection of blue pigments. The blue (actually cyan) pigment most commonly used in making color toners is C. I. Pigment 15:3, also known as Phthalo Blue A, or phthalocyanine. This pigment has a strong reflection peak at approximately 460 nm. Unfortunately, this material also has a second reflection peak at 670 nm, giving rise to some unwanted reddish tinge. A second color pigment, lacking this secondary reflectance peak in the red, is C. I. Pigment Blue 27, also known as Ferriferrocyanide, Milori Blue, Iron Blue, Bronze Blue, Prussion Blue, or Chinese Blue. This is an economical pigment of outstanding tinting strength, good brightness, and full-tone lightfastness. Unfortunately, due to the presence of two high-polarizability iron atoms in its molecular structure, this pigment is much more difficult to disperse. Indeed, based on the model described above, Blue 27 is predicted to be the most difficult of the blue pigments to disperse, with a cohesive force approximately three times that of Blue 15:3. Blue 15:3 is the most cohesive pigment that has been dispersed by conventional methods (e.g., sonication). As a result, Blue 27 has not been used in EA toners, although it is used in other commercial applications where other dispersion aids such as surfactants are not a problem.
Ultrasonic waves are often utilized in an attempt to break up particle aggregates, including color pigments. However, this is not usually very successful because the forces acting to break up aggregates occur over the length scale of xc2xd of the wavelength of the sound, the distance between local maxima and minima in the sound pressure wave. This distance is typically on the order of millimeters. On the other hand, pigment particle sizes are typically on the order of 100-800 nm, and their aggregates on the order of 0.3-1 microns, much smaller than the sizes that can be broken up by usual ultrasound techniques. These usual techniques are basically useless for the degree of pigment particle dispersion required for good chroma levels.
A need to provide a less expensive and non-chemical method for dispersing pigments to obtain good chroma levels still remains.
An aspect of the invention is to provide a method and an apparatus for dispersing aggregates in a fluid medium. The present invention employs an ultrasonic device to efficiently breakup particle agglomerates by driving the ultrasonic signal over a small range of frequencies around the acoustic slow wave frequency of the saturated agglomerate. At this frequency, the fluid vibrates out of phase with the solid and is forced through the pore structure in the agglomerate. This relative motion of fluid and solid exerts high viscous stresses at the particle-particle contact points, which leads to fracture of the agglomerate.
In another object of the present invention there are provided simple and economical methods for making toner including the steps of: dispersing pigment aggregates and wax in a solution of particle latex constituents; emulsifying the solution; blending the solution; aggregating and coalescing the solution to form toner particles; subjecting the solution to an acoustic slow wave frequency to cause pore fluid motion within pigment aggregates thereby breaking up pigment aggregates in the solution, said subjecting step is applied during one or more of said steps of: said dispersing, emulsifying, blending or aggregating; washing the toner particles; and drying the toner particles; wherein said subjecting step includes the step of: determining the acoustic slow wave frequency; and wherein said determining step includes calculating said acoustic slow wave frequency from the following equation:
fc=xcex7{Sv2(1xe2x88x92xcfx86)2}/(2xcfx80Bxcfx862xcfx81f)
Where fc is the acoustic slow wave frequency, xcex7 is the solution viscosity, Sv is the primary pigment surface area per unit volume of the aggregates, xcfx86 is the pigment aggregates porosity, xcfx81f is the solution density, and B is a constant.